Electrochemical devices, including fuel cells and electrolytic cells, generally involve producing electric current or desired chemical compounds by interconverting electrical and chemical energy. A typical electrochemical device includes an anode (fuel electrode), a cathode (air or oxygen electrode), an ion-conducting electrolyte positioned between the anode and cathode, and a separator, or bipolar plate, for separating the anode of one cell from the cathode of an adjacent cell and conducting electrical current. Fuel cells generate electrical energy by promoting the chemical reactions at the electrodes. More specifically, fuel gas is supplied to the anode surface producing an electron-generating oxidation reaction, oxygen is supplied to the cathode surface producing an electron-consuming reduction reaction, electric charge is transferred from the anode to the cathode through the bipolar plate, and internal charge transfer is accomplished through the electrolyte. Conversely, electrolytic cells produce desired chemical compounds by driving chemical changes in the electrochemical cell with the passage of electric current through the electrolyte. For example, a water electrolyzer produces hydrogen and oxygen, and chlorine production is accomplished by the electrolysis of aqueous solutions of sodium chloride and hydrochloric acid.
A plurality of electrochemical cells are generally stacked together in electrical series to produce a useful voltage. To form the cell stack, bipolar plates electronically connect the anode of one cell with the cathode of an adjacent cell. Bipolar plates may also include structures, e.g. flow fields, for evenly distributing the reactants over the active surfaces of the electrochemical cell, or the bipolar plates may provide support for such structures within the electrochemical cell. Where a liquid electrolyte is used, bipolar plates may further incorporate reservoirs for replenishing the electrolyte supply.
Bipolar plates serve as both current collectors and separation barriers within electrochemical cell stacks. As a current collector, bipolar plates provide an electrical connection between adjacent electrochemical cells, and, therefore, bipolar plates must exhibit good electrical conductivity. As a separation barrier, bipolar plates isolate the anode and cathode reactants and products within one cell from those in an adjacent cell, requiring them to be impervious to the electrochemical cell reactants and products, as well as any liquid electrolyte, and resistant to corrosion under the operating conditions of the electrochemical device. Operating conditions frequently include harshly acidic environments, high electrical potentials, and high temperatures. Ideally, bipolar plates are also easy to fabricate, physically durable, thin, and lightweight.
Bipolar plates are critical to the efficient operation of advanced commercial multi-cell electrochemical devices, such as phosphoric acid fuel cells (PAFCs), regenerative proton exchange membrane (PEM) fuel cells, water and hydrogen chloride electrolytic cells, and lead-acid batteries. In these systems, one electrode operates at a high anodic potential, in the range of between about 1.0 to about 2.0 volts, as measured against the reversible hydrogen electrode reference (vs. RHE). PAFC bipolar plates are traditionally made of graphite and carbon-based materials, which severely corrode and are thermodynamically unstable at electrode potentials of greater than 0.8 volts vs. RHE. For example, bipolar plates made from graphite resin mixtures that are carbonized at low temperatures are unsuitable and rapidly degrade under the operating conditions of PAFCs, fuel cells having an electrolyte comprised of phosphoric acid (H.sub.3 PO.sub.4). Heat treating these carbonized graphite plates reduces the corrosion current by two orders of magnitude at 0.8 volts in 97% H.sub.3 PO.sub.4 PAFCs at 190.degree. C., however, while slowed, the corrosion eventually limits the fuel cell's operating lifetime.
Corrosion problems are even more severe in more advanced electrochemical devices, such as in PEM electrolytic cells, where anodic potentials may exceed 1.8. volts vs. RHE and the bipolar plates are in constant contact with highly acidic solutions (about pH 0.0 to about pH 3.5). PEM fuel cells, also referred to as solid polymer electrolyte fuel cells (SPEFCs), utilize a solid polymer, proton-conducting membrane as an electrolyte, which is typically a perfluorinated sulfonic acid polymer electrolyte membrane. The PEM fuel cell operating environment includes strong acidic oxidizing and reducing conditions, and temperatures up to 125.degree. C. PEM fuel cell bipolar plates are typically comprised of hydrophobic carbon felt paper and graphite support material. The conducting parts of the fuel cell frames may be titanium and the non-conducting parts polysulfone. PEM fuel cells are of current and specific interest in automotive applications, as they ameliorate environmental pollution problems associated with combustion engines and offer renewable source of energy.
One approach to the problem of bipolar plate corrosion is to use materials having the ability to withstand the oxidizing conditions within the electrochemical cell, such as titanium, niobium, and tantalum. These metals, however, are prohibitively expensive. In addition, in highly acidic applications, such as PEM fuel cell applications, these metals are subject to anodic dissolution at the cathode, hydrogen embrittlement at the anode, and the formation of electronically resistive oxide films. Platinizing the metal in the active area of the oxygen electrode to maintain its conductivity is possible, however, this involves molding a graphite polymeric binder base plate and covering it with a platinized metal foil, such as tin, also a very expensive process because of the composite structure of the bipolar plate.
For electrochemical devices to become a competitive energy technology, the power densities and operating lifetimes of the devices must be increased and the manufacturing and operating costs reduced. In particular, a need continues for an easily fabricated, economic, non-corrosive, highly electronically conductive bipolar plate material for use in electrochemical devices operating at high potentials and in harshly acidic environments.
The present invention is a corrosion resistant titanium carbide bipolar plate formed by blending titanium carbide powder with a suitable binder material and molding the mixture, at an elevated temperature and pressure. The molded titanium carbide bipolar plates have long term mechanical and thermodynamic stability in acidic media at high anodic potentials and exhibit good electrical conductivity. In addition, the method of molding the bipolar plate is economically attractive and allows for greater flexibility in the design of the plate than traditional machining processes.
Therefore, in view of the above, a basic object of the present invention is to provide a bipolar plate capable of withstanding high potentials and harshly acidic environments within electrochemical cells.
Another object of this invention is to provide a bipolar plate that has good electrical conductivity and is resistant to corrosion under the operating conditions of the electrochemical device.
Yet another object of this invention is to provide a low cost, physically durable bipolar plate that is easy to fabricate.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentation and combinations particularly pointed out in the appended claims.